Gimp - GNU Image Manipulation Program

GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.
It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
GIMP is written and developed under X11 on UNIX platforms. But basically the same code also runs on MS Windows and Mac OS X.

BIMP is a graphical batch plugin to apply a set of GIMP manipulations on groups of images.
You can use predefined manipulations like resize, crop, flip, rotate, color correction, sharpen, watermark, format conversion, rename, but you can also select other installed plugins or script-fu and apply them to each image.